Pressure sensors are used in pressure measuring arrangements in almost all areas of industrial measurement technology to metrologically detect pressures to be measured.
For this purpose, so-called semiconductor sensors, such as pressure sensor chips based upon silicon, are often used today. These pressure sensors are, however, mechanically sensitive. For this reason, they cannot be exposed directly to the medium for which the pressure is to be measured. Instead, a diaphragm seal is provided upstream of the pressure sensor and transmits the pressure to be measured to the pressure sensor. Diaphragm seals are filled with a pressure-transmitting fluid, such as oil. Since the pressure-transmitting fluid freezes at low temperatures, these pressure sensors can only be used at temperatures that are far above the freezing point of the fluid used. The use of these pressure sensors is therefore limited to applications in which only temperatures above a minimum temperature dependent upon the selection of the fluid—generally above −70° C.—occur.
Extremely low temperatures below −70° C. occur in special applications, such as in systems for the liquefaction of natural gas, as well as in the transport of liquefied natural gases. In these cases, temperatures of down to −165° C. can occur.
Basically, therefore, for the measurement of pressures at temperatures below −70° C., only pressure sensors—often called dry sensors—that may be operated without the use of pressure-transmitting fluids are considered.
An example of dry pressure sensors are ceramic pressure sensors with a ceramic measuring diaphragm arranged on a cylindrical ceramic base body at a distance from said base body and having the same diameter as said base body. Ceramic pressure sensors offer the advantage that the ceramic measuring diaphragm can be exposed directly to the medium. In this respect, ceramics have particularly advantageous chemical and mechanical properties for application in pressure metrology. Ceramic pressure sensors must be installed at the location of use. To this end, they are regularly inserted into a sensor housing that is equipped with a process connection and in which an outer edge of the measuring diaphragm and base body is clamped in the axial direction, i.e., parallel to a surface normal to the measuring diaphragm. In order to avoid mechanical deformations within the pressure sensor, particularly in the area of the measuring diaphragm, the pressure sensors are clamped into the sensor housing by inserting intermediate elastomers. Elastomers, however, fail at the low temperatures mentioned. Accordingly, the use of these ceramic pressure sensors is also limited to temperatures above −70° C.
Another example of a dry pressure sensor is described in German Patent, DE 10 2006 035 230 A1. This pressure sensor comprises a sapphire carrier with piezoresistive silicon sensors mounted therein and interconnected to form a resistance bridge. The sapphire carrier is mounted on an inner surface of a titanium plate used as measuring diaphragm. During measurement operation, the pressure to be measured is directly applied to the outside of the titanium plate, and the plate's pressure-dependent bending is metrologically detected using the silicon sensors. The titanium plate is flush-mounted in a diaphragm carrier made of titanium, which in turn is inserted into a bracket that is made of a stainless steel and flush-mounted in a process connection made of a stainless steel. The diaphragm carrier is used to absorb stresses that may arise due to the different thermal expansion coefficients of the measuring diaphragm made of titanium and the bracket made of stainless steel, as well as of the process connection made of stainless steel. For this purpose, the diaphragm carrier comprises a circumferential annular recess on the inner surface facing the inside of the pressure sensor. The diaphragm carrier is thus able to protect the measuring diaphragm against deformations even in the case of rapid temperature changes. However, the carrier can do so only as long as it is itself still elastic enough due to the recess to absorb the forces caused by the different thermal expansions. These forces are all the larger, the lower the temperature is. In parallel, the lower the temperature is, the lower is the elasticity of titanium. The diaphragm carrier is thus less and less able to absorb these forces, the lower the temperature is at which it is used.
With dry pressure sensors, the distortion-free as possible mounting of the pressure sensor or the measuring diaphragm at the location of use at low temperatures poses significant problems.